The traditional technique for making ophthalmic lenses involves repeated grinding passes usually known as sweeps across a lens blank with a cutting tool, which is usually a circular cup shaped diamond tool. The sweeps are continued until the blank has been shaped to the desired surface curvature and lens centre thickness. The range of curvatures produced by available conventional lens grinding machines in the spherical meridian is in the range from 3 to 20 dioptres. It is desirable to extend that range down to zero dioptres i.e. lenses with no curvature in the spherical meridian. The extension of the range of lens grinding machines has been the subject of previous proposals e.g. U.S. Pat. No. 4,535,566 describes a mechanical system in which the locus of the grinding wheel can be varied to extend the range of the system. The basis of this proposal is to use a cam follower mechanism to radially reposition the diamond tool as it is swept over the lens blank surface. The change in the sweep radius length while sweeping simulates a particular radius of curvature. The operations described in U.S. Pat. No. 4,535,566 required a designated cam surface (or template) for every specific base curve it is desired to generate. U.S. Pat. No. 4,535,566 maintains the same head angle, i.e. the angle at which the tool head is set to a tangent to the curve being cut throughout the sweep by means of a complex four bar linkage which must be adjusted according to the desired prescription before the lens generation process begins. Thus this prior proposal provides a lens grinding machine which while having an extended range, is only adjustable to produce selected curves within that range, and requires a skilled operator to set the machine up for a particular power.
An object of the present invention is to produce a lens grinding machine and a lens generating method which can be operated with limited operator attention and the use of relatively unskilled operators in that no complex setting up procedures are required by utilising numerical control procedures. It is a further object to produce a machine with an extended range, which can be produced by relatively modest machine modifications to existing designs.